The present invention relates to an efficient and expeditious method and apparatus for forming a molded plastic article, which advantageously prepares such an article with a film as an outer layer thereof.
In accordance with U.S. Pat. No. 5,401,457 for PROCESS FOR FORMING A COLOR COATED ARTICLE, By Emery I. Valyi, Patented Mar. 8, 1995, a process is provided for forming a color coated article. In accordance with the procedure of the ""457 patent, a film is placed substantially flat over a mold cavity and deformed by a core half and by molten plastic entering though a sprue. However, the procedure of the ""457 patent is difficult to control, particularly when molding parts with large surfaces and sharply varying curvatures. The difficulties increase when the plastic is pressure molded at high enough temperatures to reduce the film strength substantially, as is the case with conventional injection molding procedures.
In accordance with U.S. patent application Ser. No. 09/130,864, filed Aug. 7, 1998, by Emery I. Valyi et al., a color coated article is prepared by depositing molten plastic on a film and forming the film-molten plastic combination in a mold cavity. The molten plastic is delivered either by a curtain from a traversing linear die or as discrete piles or globs from valves in a hot runner manifold or shooting pot. However, it is desirable to improve upon this procedure. In accordance with this procedure, the die coverage is fair to good for rectilinear parts, but poor for parts with significant curvature,""such as wheel covers or automotive parts as fenders. The correction for this with die delivery is to traverse the die laterally, but this is prone to air entrapment as the resin tends to fold on itself. However, to vary the width of the die gap creates a complex die design and a complex control problem. In addition, there is a time delay from the first resin deposit on one film edge until completion of the deposit on the distant film edge.
Valve delivery (hot runner manifold or shooting pot) is an improvement over die delivery for curved parts. Valve delivery allows all of the resin to be deposited at the same time. This is accomplished by using a number of valves and associated electrical connections and controls. However, lateral flow of molten resin deposited in a pile is very slow for most polymers at deposition temperatures. Thus, many valves are needed for uniform coverage, otherwise gaps exist between piles until the piles are forced together at mold closure. This creates problems, such as knit lines and hot spots. Knit lines create structural failure points and hot spots may create adverse visual patterns on the external surface.
It is, therefore, a principal object of the present invention to provide an improved method and apparatus for preparing molded articles.
It is a further object of the present invention to provide a method and apparatus as aforesaid which efficiently and expeditiously prepares curved molded articles, and enables the preparation of such articles with an outer film layer.
Further objects and advantages of the present invention will appear hereinbelow.
In accordance with the present invention, the foregoing objects and advantages are readily obtained.
The method of the present invention comprises: providing a mold cavity; placing a template over the mold cavity which includes a series of orifices which extend over substantially the entire surface of the mold cavity; passing molten plastic through said orifices in a pattern corresponding to the shape of the article to be molded; and subsequently forming said molten plastic into a molded article having the shape of the mold cavity. The molten plastic may be transferred from the orifices directly to the mold cavity. However, desirably a film is placed over the mold cavity, particularly a colored film, the molten plastic is deposited from the orifices onto the film in said pattern, and the film-molten plastic combination formed in the mold cavity into a molded article having the shape of the mold cavity, wherein the film is an outer layer of the molded article.
In a preferred embodiment, a gate control plate is placed adjacent the template having closed gate areas and open gate areas, the gate control plate is moved into a first closed position wherein the closed gate areas block the orifices and prevent molten plastic from passing therethrough, and into a second open position wherein the open gate areas register with the orifices and allow molten plastic to flow through the orifices.
The apparatus of the present invention comprises: a mold cavity; a template positioned over the mold cavity which includes a series of orifices which extend over substantially the entire surface of the mold cavity; means for transferring molten plastic through the orifices in a pattern corresponding to the shape of the article to be molded; and means for forming the molten plastic into a molded article having the shape of the mold cavity.
Further features and advantages of the present invention will appear hereinbelow.